In a preliminary study of 152 people, the test correctly identified 98% of those who had had an ischemic stroke and 86% of those who hadn't had one. Ischemic stroke is the most common type of stroke. It occurs when a clot blocks blood flow in the brain.

The test measures levels of a brain chemical called glutamate. When blood flow to the brain is impaired, glutamate is rapidly released into the bloodstream, says researcher Kerstin Bettermann, MD, PhD, associate professor of neurology at Penn State College of Medicine in Hershey.

The findings were presented here at the annual meeting of the American Neurological Association.

Imaging Scans Used to Diagnose Stroke

Each year, more than 750,000 Americans have a stroke. About 80% are ischemic strokes. The rest are hemorrhagic stroke, caused by bleeding in the brain.

Currently, there is no blood test for detecting stroke, although several groups are developing them.

When a person arrives at the hospital emergency room with stroke symptoms, a CT or MRI scan of the head is used to help doctors determine if a stroke is in progress and if the person is a candidate for clot-busting drugs.

But a CT scan doesn't always produce a clear enough image of what is going on in the brain, Bettermann tells WebMD. "An MRI is what is needed, but not all medical centers have them. And some patients can’t have an MRI -- if they have a pacemaker or are claustrophobic, for example."

That's where the blood test comes in, which Bettermann estimates would only cost about $20.

It wouldn't replace imaging tests and doctor exams, she says, but used as an add-on to standard tests, it would improve diagnosis.

Glutamate Levels Rise in Stroke Patients

The new study included 50 people with ischemic stroke and 102 people without stroke. In the group without stroke, 48 people had stroke risk factors such as diabetes or high blood pressure but no history of stroke, 28 people had stroke-like symptoms such as weakness on one side of the body but were not having a stroke, and 26 were healthy.

Glutamate levels were measured in their blood within 24 hours of their first symptoms (or in the case of people without symptoms, within 24 hours of entering the study).

Patients were given head CT scans and, in most cases, MRI scans as well, to confirm whether they were having a stroke.

After just one hour, blood levels of glutamate began to rise in patients having a stroke, Bettermann says.

"There was a clear difference in glutamate levels between ischemic stroke patients and all the other groups," she says.

Imaging Still the Gold Standard

Ralph L. Sacco, MD, head of neurology at the University of Miami Miller School of Medicine and immediate past president of the American Heart Association/American Stroke Association, tells WebMD that development of an accurate blood test to detect ischemic stroke continues to be an important goal.

Such a test could accelerate diagnosis and treatment for stroke, he says.

"This [test] seems promising, but more work is needed to make sure we are not missing cases. We also need to know if [glutamate levels] increase in people having hemorrhagic strokes and transient ischemic attacks (TIAs)," Sacco says.

For now, brain imaging is still the diagnostic tool to diagnose stroke, he says.

Bettermann says the next step will be to validate the findings in a larger study.

These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.